Toby Melville, Reuters

Police officers stand guard beside a cordoned-off area, after former Russian military intelligence officer Sergei Skripal, who was convicted in 2006 of spying for Britain, became critically ill after exposure to an unidentified substance, in Salisbury, southern England, March 5, 2018. Reuters

* Former Russian spy was in Britain after 2010 swap

* Russia convicted the man of treason in 2006

* Police say unidentified substance to blame

* Pizza restaurant closed in Salisbury

SALISBURY, England - A former Russian double agent convicted in Moscow of betraying dozens of agents to British intelligence was critically ill in hospital on Tuesday after he was exposed to an unidentified substance in southern England.

Sergei Skripal, once a colonel in Russia's GRU military intelligence service, was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the West as part of a Cold War-style spy swap on the tarmac of Vienna airport.

But the 66-year-old former spy and a 33-year-old woman who was known to him were found unconscious on a bench in a shopping centre on Sunday in the English city of Salisbury after exposure to what police said was an unknown substance.

Both were critically ill in intensive care.

While the British authorities said there was no known risk to the public, police sealed off the area where the former spy was found and a pizza restaurant called Zizzi in the centre of Salisbury. Some investigators wore yellow chemical suits.

British police did not release the names of those who were being treated but two sources close to the investigation told Reuters that the critically ill man was Skripal. It was unclear what the substance was, they said.

"This has not been declared as a counter-terrorism incident and we would urge people not to speculate," Wiltshire police Temporary Assistant Chief Constable Craig Holden told reporters.

"I must emphasise that we retain an open mind, and that we continue to review this position."

Relations between Britain and Russia have been strained since the murder of ex-KGB agent Alexander Litvinenko with radioactive polonium-210 in London in 2006, a killing which a British inquiry said was probably approved by President Vladimir Putin.

The Kremlin has repeatedly denied any involvement in the killing of Litvinenko.

Litvinenko, 43, an outspoken critic of Putin who fled Russia for Britain six years to the day before he was poisoned, died after drinking green tea laced with the rare and very potent radioactive isotope at London’s Millennium Hotel.

It took some time for British doctors to discern the cause of Litvinenko's illness.

Another Russian, Alexander Perepilichny, who had been helping a Swiss investigation into a Russian money-laundering scheme, was found dead in 2012 in Britain. Police ruled out foul play despite suspicions he might have been murdered with a rare poison. An inquest has yet to give a definitive conclusion as to how he died.

SPY SWAP

Skripal was arrested in 2004 by Russia's Federal Security Service (FSB) on suspicion of betraying dozens of Russian agents to British intelligence. He was sentenced to 13 years in prison in 2006 after a secret trial.

Skripal, who was at the time shown wearing a track suit in a cage in court during the sentencing, had admitted betraying agents to MI6 in return for money, some of it paid into a Spanish bank account, Russian media said at the time.

But he was pardoned in 2010 by then-President Dmitry Medvedev as part of a swap to bring 10 Russian agents held in the United States back to Moscow.

The swap, one of the biggest since the Cold War ended in 1991, took place on the tarmac of Vienna airport where a Russian and a U.S. jet parked side by side before the agents were exchanged.

One of the Russian spies exchanged for Skripal was Anna Chapman. She was one of 10 who tried to blend in to American society in an apparent bid to get close to power brokers and learn secrets. They were arrested by the FBI in 2010.

The returning Russian spies were greeted as heroes in Moscow. Putin, himself a former KGB officer who served in what was then East Germany, sang patriotic songs with them.

Skripal, though, was cast as a traitor by Moscow. He is thought to have done serious damage to Russian spy networks in Britain and Europe.

The GRU spy service, created in 1918 under revolutionary leader Leon Trotsky, is controlled by the military general staff and reports directly to the president. It has spies spread across the world.

Since finding refuge in Britain, Skripal lived quietly in Salisbury and kept out of the spotlight until he was found unconscious on Sunday.

"On the bench there was a couple, an older guy and a younger girl. She was sort of leant-in on him. It looked like she'd passed out maybe. He was doing some strange hand movements, looking up to the sky," witness Freya Church told the BBC.

"They looked so out of it that I thought even if I did step in I wasn't sure how I could help, so I just left them. But it looked like they'd been taking something quite strong." (Writing by Guy Faulconbridge, Additional reporting by Alistair Smout, William Schomberg, Andy Bruce and Michael Holden in London and Mark Hosenball in Washington Editing by Nick Macfie and Raissa Kasolowsky)